1. Field of the Invention
The present invention relates to a head driving device in a disk drive which records, reproduces and erases information using light or magnetism.
2. Description of the Prior Art
FIG. 1 is a schematic side view of a conventional optical disk unit. In this conventional optical disk unit, recording, reproducing and erasing of information are performed optically while a disk 1 is rotated at high speed by means of a motor 2 and an optical head 3 is moved radially of the disk. In order to ensure recording, reproducing and erasing of information at high speed and high density, it is necessary to use a head driving device of high speed and high accuracy and realize a highly stable servo system.
FIG. 2 is a sectional view of a conventional head driving device which is disclosed, for example, in Japanese Patent Publication No. 27945/88. An optical head 3 is carried on a carriage 4, and voice coils 5a and 5b are attached to both side faces of the carriage 4. The carriage 4 is provided with bearings 7a and 7b which engage linear guide rails 6a and 6, respectively, and guide the carriage radially of a disk 1.
The voice coils 5a and 5b are incorporated in a twin body type magnetic circuit for access which comprises linear motor yokes 8a, 8b and permanent magnets 9a, 9b. The linear motor yokes 8a (or 8b) are in the form of a loop having a plurality of parallel sides which are long in the moving direction of the carriage 4, with one side thereof being inserted in the voice coil 5a (or 5b). On the other side opposed to the outside face of the voice coil 5a (or 5b) is disposed a permanent magnet 9a (or 9b).
In the head driving device of such construction, when a predetermined electric current is allowed to flow through the voice coils 5a and 5b, a force acting in the radial direction of the disk 1 is applied to the carriage 4 due to the action of magnetic lines of force generated in the magnetic circuit, so that the carriage moves radially of the disk together with the optical head 3. In this case, if the device is designed in such a manner that a central working line of the forces exerted on the two voice coils 5a and 5b is at the same height as the center of gravity of the whole of a carriage assembly (a moving portion), a pitching vibration is not generated in the moving portion because there is not generated a turning moment around the center of gravity of the moving portion, so that a highly stable tracking servo system can be realized.
In such conventional head driving device, when thrust is applied to the voice coils, since a central working line of the forces exerted on the voice coils is deviated from the center of gravity of the moving portion, there is generated a turning moment around the centroid of the moving portion, so that there is excited a pitching resonance of a mass-spring system which is formed by both the inertia of the moving portion and the spring characteristics of the bearings.
As stated above, however, in the case where the position of the central working line of the forces applied to the voice coils relative to the centroid of the moving portion is deviated as in a head driving device according to a first aspect of the present invention, the tracking servo system does not become unstable in a pitching resonance portion which appears on open loop frequency characteristics of the whole of the tracking servo system because antiresonance and resonance appear in this order from the low frequency side, thus affording a phase leading direction. Further, since it is not necessary to make the alignment of the force working center line with the center of gravity of the moving portion strictly, the design and the maintenance of parts are no longer complicated.
The above and further objects and novel features of the present invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for purpose of illustration only and are not intended as a definition of the limits of the invention.
However, in order to suppress the pitching vibration of the moving portion, it is necessary to perform fairly strictly the alignment between the central working line of the forces exerted on the voice coils 5a and 5b and the center of gravity of the moving portion, thus giving rise to the problem that the operation in designing an actual machine and the maintenance of high machining and assembling accuracy for the components which constitute the moving portion are complicated.